Examples of various fuels that can be utilized as secondary fuel include car tires, railway sleepers, furniture, carpets, wooden waste, garden waste, kitchen waste, paper sludge, biomass, petcoke, wastewater sludge, meat and bone meal, fuller's earth, by-products from other industries and not finely ground coal.
The aforementioned method is known from the European Patent No. 0 105 322 B1. This patent gives a description of a method for manufacturing cement clinker where cement raw meal is heated in a preheater, introduced and burned into clinker in a rotary kiln and subsequently cooled in a clinker cooler. The heated cement raw meal and a secondary fuel are introduced to the rotary kiln in the same area and heated by contact with hot gases formed by burning of a primary fuel in the rotary kiln. Since the secondary fuel and the cement raw meal are introduced to the same area of the rotary kiln, the cement raw meal will be in considerable contact with the reducing zones created in connection with the combustion of the secondary fuel. Reducing zones are created when the stoichiometric ratio between oxidants (e.g. O2) on the one hand and fuel and intermediate products from the combustion (such as free carbon, CO and H2) on the other hand is such that the amount of fuel exceeds the amount of oxidants. Such reducing zones will always occur locally around a fuel particle and around combustible gases and liquids. According to the invention described in the European patent, there will be considerable contact between the cement raw meal and the reducing zones, entailing a number of disadvantages. Firstly, in areas of the kiln system with a counterflow between gases and the predominant solid matter portion, which is typically the case in the rotary kiln, the result will be material cycles where components are released from the cement raw meal to the gas phase in one zone of the kiln plant for subsequent re-absorption in the cement raw meal in another zone of the kiln plant and then directed back. For example sulfur may be part of such a cycle. Sulfur primarily takes the form of CaSO4 or CaSO3 in the cement raw meal. CaSO4 is reduced by the following reactions (and other similar reactions).CaSO4(s)+C(s)→SO2(g)+CaO(s)+CO(g)CaSO4(S)+CO(g)→SO2(g)+CaO(s)+CO2(g)
The sulfur is re-absorbed in the form of CaS (possibly CaSO3) when the gases are cooled and brought into contact with CaO/CaCO3 as for example in the lowermost cyclone stages. This will cause sulfur to be accumulated in the system between the preheater and the reducing burning zone. Other occurrences, in addition to sulfur, will be material cycles with i.a. halogens (Cl, Br, F), alkali compounds (Na, K) and Mg, Pb, and Cd. Separately and combined the material cycles may give rise to increased build-up of coatings in the system, primarily in the riser duct. Also the flow properties of the solid matter may undergo changes in response to these cycles, e.g. resulting in cyclone blockages. It is desirable to avoid such coatings and the mentioned changes of flow characteristics since they will lead to build-up of material and blockage problems in the plant.
A second problem with reducing zones in the cement raw meal is that metals such as e.g. Fe and Cr will be reduced. For example Fe can be reduced according to the reaction indicated below.Fe(III) reducing conditions→Fe(II)
Reducing metals may adversely affect the quality of the finished product and, therefore, they should be avoided.
In addition to the aforementioned disadvantages, there is also a risk that the cement raw meal when mixed with secondary fuel will deposit on the surface of the fuel, thereby completely or partially restricting the substance transport between gases and secondary fuel, resulting in a reduction of the fuel conversion rate.